The invention relates to a passive polarization stabilizer and applications thereof in optical systems and sub-systems.
Polarization stabilization is required in many applications. For example, low-loss telecommunications optical fiber is not polarization preserving. Consequently, light emerging from such fiber will have an undefined polarization state. On the other hand, many devices which may be arranged to receive light from an optical fiber are sensitive to the polarization state of the input light. Examples of such polarization sensitive devices are semiconductor optical amplifiers (SOA's), interferometric fiber sensors, electro-optical modulators, wavelength division multiplexing couplers and electro-optical switches based on electro-optic crystals or liquid crystals. The use of polarization preserving fiber can solve the problem, but is often unattractive since polarization preserving fiber is relatively costly and has higher loss, and may not even be possible for many telecommunications applications where devices must be compatible with existing installed fiber.
To provide polarization stabilization, many types of active polarization stabilizers are available. By active it is meant that an element of the device is controlled according to a feed-back (or feed-forward) signal derived from a signal indicative of the polarization state. Active polarization stabilizers are known that control a birefringent element or a Faraday rotator based on a feed-back signal. However, for many telecommunications networks, active devices are to be avoided if possible in view of reliability issues and the need to supply power to the device.
Passive polarization stabilizers would thus, in principal, be attractive devices for many applications. For example, a passive polarization stabilizer could be used as a component in a passive optical network. Generally speaking, a passive device can be expected to be more reliable and less complex and costly than a comparable active device.
At the present time, the inventors know of no commercially available passive polarization stabilizer.
A polarization stabilizer is proposed in JP-A-8248358 (reference [1]). However, it is not understood how this proposed device could possibly function in practice, because its design does not take account of the fact that the phase difference between the two beams at the end of the arms (optical transmission lines 3-1, 3-2) is dependent on the input polarization state. The interference of these beams on recombination (at optical muliplexer 2) would produce an output power that is dependent on the input polarization state. The polarization stabilization function would therefore not be achieved.